1. Field of the Invention
The present invention relates to an optically functional device for use as optically operational devices and memories in optical information processing apparatuses, particularly relates to an optically functional device which is applicable to information processing apparatuses having image processing functions and neutral net functions operated with light, or which may be applied to various controlling apparatuses in use of these functions.
2. Description of the Related Art
Performing arithmetical operations and memory operations using light requires so-called optically functional devices, or the devices in which a light output is emitted in a nonlinearly responding manner in accordance with a light input.
As a first referential example, there has been proposed an optically functional device configurated as shown in FIG. 7 (a document: refer to J. Lightwave Technology vol. LT-3 (1985) 1264). This optically factional device is one, as detailed in the aforementioned document, in which a so-called thyristor structure consisting of pnpn-layers is formed.
With respect to this device, there are provided a light emitting portion in the upper portion comprising three layers, (specifically, p-InP Confining Layer, N-InGaAsP Active Layer, and n-InP Confining Layer), a transistor (HPT; Hetero Bipolar Transistor) in the lower portion comprising four layers (n-InP, n-InGaAsP buffer, p-InGaAsP Gate, and n-InP Emitter) and a pair of metal layers, functioning as electrodes, being disposed on the upper and lower surfaces of the device. As to this device, with the state that electric field is applied between the electrodes, when light is inputted from the rear side of the substrate, the HPT is turned to ON to flow current, thus causing the light emitting portion to emit light upward. At this time, a part of the emission is inputted into the HPT, this light becomes a so-called feedback light, causing a nonlinear operation.
FIG. 8 shows the operation of the device schematically. In FIG. 8, the current is taken along the axis of ordinate, the applied voltage is taken along the axis of abscissa, the thyristor characteristic of the pnpn-structure is represented with a solid line, and the operational line based on the load resistance existing in the system is denoted by broken line.
In FIG. 8, with increase in the input light intensity, the break down voltage V.sub.B changes in the order of 1.fwdarw.2.fwdarw.3, furthermore, in accordance with this change, the state of intersection points with the operational line also changes from two points P and Q to a sole point Q. More, specifically, in a case where the input light intensity corresponds to the state of 1 or 2, as is apparent there exit two stable points forming a so-called bistable state. (In this figure, V.sub.A denotes the applied voltage, and R.sub.L the load resistance value.)
Consequently, as shown in FIG. 9, the nonlinear operations, that is, respective characteristics such as differential gain (a), bistability (b) and optical switch (c), can be obtained in accordance with applied voltages. Further, the respective characteristics can be acquired when the value of the load resistance is varied.
FIG. 10 is an operational principle diagram showing aforementioned optically functional device by using an equivalent circuit. In the figure, reference numerals 21, 20, and 22 represent the HPT, the light emitting device and the load resistance, respectivly, meanwhile, reference numerals 24 25 and 26 denote respectively the feed back light, the output light outward, and the input light into the HPT.
As a second referential example, there is also a device which is disclosed in "Technical Digest, 20C3-2, Integrated Optics and Optical-fiber Communication (IOOC), 1989, Kobe, Japan." This device has mostly the same structure with the first referential example. Either of these examples needs to be operated by connecting in series with an appropriate load resistance. At this time the load resistance value is to be selected in accordance with the input light intensity and applied voltage used and the desired character.
For optically functional devices in addition to the aforementioned two examples, there is a third kind of structure, which is shown in a patent application No. 73908/1990 applied by the present inventor. The structure of the device of this prior application is shown in FIG. 11. The operation and operational principle of the device based on this preceding invention is the same as the aforementioned referential examples, and the behaviors of its operation are as shown in FIGS. 8 and 9, and the equivalent circuit is expressed in FIG. 10.
The device of this invention is an optically functional semiconductor device which has a light receiving portion I disposed on a semiconductor substrate 2, a light emitting portion II thereon, and which is equipped on the side of the light emitting portion with a window 10 through which the input light and output light goes in and out. In this arrangement, the light emitting portion II is made of a semiconductor material having an energy of forbidden band width of more than the energy of a main peak of input light, and the light receiving portion I is made of a semiconductor material having an energy of forbidden band width equal to or less than the energy of a main peak of input light, and the optically functional device characteristically receives and feeds back by means of the receiving portion I a part of output light generated from the light emitting portion II and performs a nonlinear output light response to input light based on the feedback effect of the absorbed light by means of the light receiving portion I. Consequently, The light emitting portion can be commonly used as the input window, thus making it possible that input light and output light can be respectively, received by and emitted from, the same position. Furthermore, it is possible that the wavelength of the input light can be differed from that of the output light, so that input light can be easily separated.
In FIG. 11 is a sectional view showing a structure of the aforementioned optically functional device, the layers structures are formed of, from the bottom in the order, a rear electrode 1, an n-type GaAs-substrate 2, an n-Al.sub.0.4 Ga.sub.0.6 As layer 3, a p-GaAs layer 4, an n-GaAs layer 5, an n-Al.sub.0.4 Ga.sub.0.6 As layer 6, a p-Al.sub.0.4 Ga.sub.0.6 As layer 7, a p-GaAs layer 8 and another electrode 9. In this arrangement, the portion I serves as a light receiving portion (3, 4 and 5) being formed as an HPT, and the portion II works as a light emitting portion (6 and 7). Reference numeral 10 denotes the input/output window of light, and the progressing directions of the input and output light are shown by arrows 11. The device of this example is also required to be connected to an appropriate load resistance in series for operation. At this time, the value of the load resistance is selected in accordance with the input light intensity and applied voltage used and the desired characteristic. This may be understood from the fact that the gradient of the operational line as shown with a broken line varies depending upon the value of the load resistance. For example, if the value of the load resistance increases and thus the gradient becomes small (the line becomes laid down), the bistable state which has two stable points P and Q comes to exist even for a low current, and simultaneously, the current difference between P and Q, or specifically, the light output difference between ON state and OFF state (ON-OFF ratio) becomes small.
Now, in the case where the optically functional device according to the aforementioned referential example is to be operated, a load resistance having an appropriate resistance value is connected in series to the device for the purpose of acquiring a desired performance. Particularly, when devices are set in two-dimensional array arrangement, each device is required to be connected with a load resistance. This is because that in a case where a common load resistance R.sub.L is connected to the optically functional devices-array as shown in FIG. 12, if any one of the optically functional devices is turned to ON, the rest optically functional devices cannot be applied with required voltages, and thus cannot be brought into operation.
It is impossible, however, that load resistances are one by one joined as in an after-treatment to the array of respective optically functional devices of some tens to some hundreds microns (.mu.m) in diameter, so that the resistance value is naturally determined by the resistance in total of the semiconductor substrates and semiconductor layers forming the light-emitting portion and light-receiving portion. This is because that in order to achieve required performances for the light-emitting portion and the light-receiving portion, the layers forming the respective portions have to be optimized in their compositions and carrier densities. For this reason, the resistance value cannot be taken as an independent parameter, and this fact limits the resulting device in its performance and operation, thus diminishing the freedom in device-designing.